As a conservationist who has been ringing the alarm bells on climate change threats to biodiversity for more than 25 years, I hardly know where to start in responding to the findings of the newest, and most alarming, Intergovernmental Panel on Climate Change (IPCC) Special Report on the impacts of a 1.5°Celsius global warming. I’m not surprised that the IPPC delivers more bad news after reviewing more than 6,000 recent scientific reports, but I am surprised by just how bad the news is.
Half of Australia’s great barrier reef bleached in recent back-to-back extreme heat episodes. Climate change of more than 1.5°C will be lethal for coral reefs worldwide. Credit: NASA.
There’s no way to sugar coat this. We’ve ignored all the warnings. We’ve ploughed on with the use of fossil fuels in the face of overwhelming evidence that this was not just a bad choice, but the worst one. And now we’ve brought ourselves to the brink of ecological catastrophe. Half a degree of additional global warming, on top the 1°C we’ve already caused doesn’t sound like much but the truth is that it could be absolutely devastating for many ecosystems and global biodiversity. For the world’s coral reefs at least, failing to limit warming to 1.5°C will be fatal, and even at that level of warming, the new IPCC report concludes that “the majority of warmer water coral reefs that exist today (70-90%) will largely disappear”. Many other biomes, ecosystems and species will also not survive, and the natural world as we know it will be irreversibly changed.
Despite the Paris Agreement’s target of keeping temperature rise below 2°C, and ideally as low as 1.5°C, all indications are that the world is barreling towards at least 3°C by 2100, and potentially significantly more, unless rapid and aggressive global actions are taken to reduce carbon emissions.
Last chance for coral reefs
More than 500 million people rely on coral reefs for food, fisheries and storm protection. The reefs, home to at least a million species of fish, invertebrates and birds, also support a globally important tourist economy and critically important cultural context for many communities and indeed whole societies. Recent massive die-backs of corals in Australia’s iconic Great Barrier Reef, resulting from back-to-back coral bleaching events driven by extreme water temperatures led scientists to the conclusion that they had underestimated climate impacts on coral reefs in previous assessments. At 2°C say the IPCC authors, the evidence suggests that tropical shallow-water “coral dominated ecosystems will be non-existent”.
For many of us, protecting coral reefs and the millions of people who rely on them would be reason enough to take the hard actions that the latest IPCC’s emissions pathways analysis shows us will be necessary to rapidly and radically rein-in carbon emissions, but this new IPCC report gives us myriad other reasons to act. Aside from the catastrophic impacts predicted for coastal communities, agriculture and human health, the broad-scale implications for natural ecosystems and global biodiversity are shown to be huge.
Many ecosystems are at greater climate risk than we realized
The impacts for many ecosystems are much worse than many scientists expected when they embarked on the Special Report. For example, in addition to coral reefs, recent die-offs in mangroves (already highly threatened by deforestation and coastal development) have led scientists to realize that they had underestimated the climate threat for this vital ecosystem too. Coastal seagrass beds and kelp forests, already impacted by climate change, will be even more severely threatened by warming above 1.5°C. According to the IPCC, sea urchins which were able to reach Tasmania from mainland Australia as a result of climate-driven changes in ocean currents have already devastated kelp populations there, and the risk to kelp worldwide is growing rapidly.
Kelp forests are keystone species for marine ecosystems but their future will be in question in many places if we exceed 1.5°Celsius of global warming. Credit: Chad King/NOAA
Also of major concern are Arctic & Antarctic marine ecosystems. In the Arctic, where ecosystems are already being transformed by warming that has been higher than elsewhere on the planet, the likelihood of experiencing ice-free summers is ten times higher at 2°C than at 1.5°C. The combination of warming and ocean acidification (which changes the carbonate chemistry of the water and particularly affects species that use calcium carbonate for their shells and/or skeletons) is already impacting bivalve mollusks (such as oysters, clams and mussels), pteropods (pelagic mollusks also know as “sea butterflies) and Antarctic krill, a crucial element of polar food webs, and vitally important for whales, as well as for fish and seabirds. According to the IPCC, at temperatures above 1.5°C, the risk to bivalve mollusks shifts to high or “very high.”
Species such as this Costa Rican rufous motmot in will be at risk if 20% or more of Central America’s tropical rainforest is lost as result of climate change. Credit: Ian D. Keating.
What a difference half a degree makes!
Globally, the damage to ecosystems would be markedly more severe at 2°C in contrast to 1.5°C. Almost twice as much of the globe’s land area is projected to experience changes in biomes (major habitat types: e.g. tundra, tropical forest or desert) at 2°C (13%) than at 1.5°C (7%). For example, tropical rainforest in Central America would be reduced by 20% at 1.5°C, 30% at 2°C, and loss would rise to a staggering 50% at 3°C – most likely being replaced by savanna and grassland. New and novel biomes will likely form as the species assemblages in current biomes breakup due to climate change, and species shift at different rates, or become extinct in response to climate change. The IPCC highlights regional high risk of warming above 1.5°C to “tropical and desert ecosystems in Asia, Australian rainforests, the fynbos and succulent karoo areas of South Africa, and wetlands in Ethiopia, Malawi, Zambia and Zimbabwe”. Largely because of drying and increased wildfire impact, South Africa’s unique fynbos biome, awarded World Heritage status for its extraordinary floral diversity and related endemic birds and insects, is predicted to lose more than twice as much area at 2°C (45%) than at 1.5°C (20%). At 3°C scientists say 80% of the fynbos would be gone.
Also since the 2014 IPCC report, the danger to insect populations has become better understood, and the new report concludes that nearly three times as many insect species may lose more than half their current habitat at 2°C (25%) than at 1.5°C (9%). The implications for ecosystem functionality could be huge, given insects’ key role in ecosystem processes such as nutrient cycling and pollination.
Tipping points revealed
Some alarming ecological tipping points are also identified in the Special Report. For example, one highlighted study shows that the biodiversity and productivity functions of the prairie pothole ecosystem of the U.S. and Canada would be preserved at 1.5°C, but not at 2°C. The prairie pothole region of the Great Plains is home to 50% of North America’s waterfowl and its wetlands provide key ecosystem services including groundwater recharge, filtering of pollutants and erosion control. There is a threshold in the Mediterranean region, where only limiting warming to 1.5°C can prevent ecosystem changes “unprecedented in the last 10,000 years”. For boreal forests (the forests that survive in the harsh, cold and nutrient poor conditions of the far north), a tipping point is thought to lie between 3°C and 4°C, a threshold that will be reached much earlier at higher latitudes than further south (NB. Regional impacts of a global average warming of 1.5°C or 2°C will vary and some regions will warm more than others). Dieback of boreal forests would also likely trigger positive feedbacks as warming and drying drive larger and more intense fires, thereby increasing carbon emissions. Boreal forest will eventually undergo a landscape-scale conversion into open woodlands or grassland.
The new IPCC assessment has revealed just how big the difference between 1.5°C and 2°C for ecosystems and biodiversity truly is. For me, the message of the science is that we have consistently underestimated the magnitude and speed of ecological impacts in the past, and that new and alarming feedbacks, tipping points and thresholds continue to be revealed as we learn more. Our understanding of the complexity of these changes is growing fast, but we already know (and have known for at least 20 years) more than enough to justify taking the political action that is needed to transform our energy systems away from fossil fuels. Keeping warming to 1.5°C is the tallest of tall orders, but it is the minimum we should aim for if time is not to run out for many of the earth’s natural systems as we know them today. As my UCS colleague, climate scientist Peter Frumhoff notes “If we care about Earth’s future on a time scale of our grandchildren’s grandchildren, we should be looking into ways to keep global temperature rise even lower than the lower bound of the Paris temperature targets.”